VOL. LXX.J PHILOSOPHICAL TRANSACTIONS. 705 



that day, a fall of snow to the depth of 12 inches. Next day the cold con- 

 tinued to increase, but so gradually, that at sun-set Fahrenheit's thermometer 

 pointed only to 21°. About midnight, a very accurate thermometer hung out 

 at a high north window, soon after pointed to 6°. At this time the air was very 

 still and serene, and the barometer stood at 30 inches. 



Thursday morning, Jan. 13, thermometer pointed as here annexed: 



At 6 o'clock, this morning Mr. W. carried 



the thermometer over to the Observatory Park, ^' ' o'clock. +6; 



and there laid it down on the snow, when the sj- +4 



mercurv sunk to 13° below O. ~, "{"',! 



"^_ •* +o 



At this time he thought it unnecessary to 4-1. +v; 



stay abroad so long in the cold as to try the ^ "'"'^ 



temperature of the air by hanging up the ther- 

 mometers, especially as he imagined that this had been done more readily, and 

 as truly, by taking the degree from the surface of the snow which had been 

 exposed to the open air during the night : but reflecting afterwards on the snow 

 at the observatory being so much below O, the greatest cold of the air at the col- 

 lege, and having on other occasions found a difference of only 4° at most in air at 

 these two stations, Mr. W. was led into a suspicion that the snow might perhaps 

 have been so far cooled down by an evaporation at the surface. With a view to 

 this opinion, he projected the experiment with the bellows described below, by 

 which he was not without expectations of producing a still more remarkable fall of 

 the thermometer when lying on the snow. All the afternoon the cold was very 

 intense, and at 7 o'clock at night the thermometer at the high north window 

 pointed to O. At 8 Mr. W. repaired to the observatory, and made choice of a 

 station at a sufficient distance from the house, and to the windward, as a light 

 air was felt coming from the east. Here he laid down two thermometers on the 

 snow with their balls half immersed, and hung up other two freely exposed to 

 the air at two feet and a half above the surface. In the following observations, 

 the interruption of the series from 2^ to 64- o'clock, was owing to an accident 

 having befallen one of the thermometers while the other was employed in the 

 trials, of which an account is subjoined. 



Thursday evening, Jan. 13, the two thermometers pointed at the degrees 

 below 0, as in the following table, at the times annexed. 



VOL. XIV. 4 X 



